1. Field of the Invention
The present invention relates to an integrated circuit buffer having a capacitive shunt circuit that reduces the noise generated by the buffer.
2. Description of the Prior Art
Integrated circuit output buffers are used to drive a load connected to an external conductor. This load typically includes a relatively large capacitance, and in some cases a significant resistive load also. For this purpose, in designing an output buffer, the output transistors are usually sized to be sufficiently large so as to drive the load at the maximum desired switching speed under worst case conditions. Such worst case conditions typically include a "slow" process, which is a deviation form the nominal fabrication process that yields an integrated circuit that operates at the low end of the acceptable speed range. In addition, operation at high temperature, and low power supply voltage, are also worst case conditions for most designs. However, compensating for these worst case conditions in the design of the buffer may result in unacceptably high switching noise. Such noise may be due to capacitive or inductive coupling from the buffer circuitry to other circuitry on the integrated circuit, or onto external conductors connected thereto. It may also be due to power supply ground bounce due to the current flow as the buffer switches.
Various techniques have recently been developed to control buffer noise. One technique is to control the drive signal to the buffer, so as to reduce the drive level as processing variations, temperature changes, or power supply voltage variations tend to increase the switching speed. In that manner, the buffer may still be designed to operate properly under worst case conditions. Then, under best case conditions, the switching speed, as determined by the rise and fall time of the buffer output voltage, remains relatively constant. Hence, the noise generated by the buffer remains relatively constant. One such technique is described in U.S. Pat. No. 4,823,029 co-assigned with the present invention. However, in some cases other techniques are desirable to control buffer switching noise.